1. Field of the Invention
The present invention relates to a heater drive circuit for driving a fixing heater used for a laser beam printer and an electrophotographic copying machine.
2. Related Background Art
A glass tube heater in which a glass tube is filled with a gas and an exothermic conductor is heated in this gas environment, has hitherto been often used as a heating means of a fixing heater utilized for a laser beam printer and an electrophotographic copying machine. In particular, a so-called halogen heater involving the use of a halogen gas as the above gas is widely utilized. This glass tube heater functions electrically as a non-linear device and has such a characteristic that an electric resistance is low in a state where a temperature of the heater is low and rises when the heater is heated. This characteristic leads to an increase in rush current when heater is switched ON/OFF.
Generally, TRIAC defined as an AC (Alternate Current) ON/OFF device is broadly utilized as a device for driving the heater. A thermistor for detecting a temperature is attached to a fixing unit, and a device for controlling the fixing unit switches ON/OFF the TRIAC in a way that detects a temperature of the thermistor. None of problems arise while the heater is kept heated, however, if switched ON in state where the heater is cooled, an excessive current flows to the heater and the TRIAC due to the non-linear characteristic of the heater. Incidentally, the rush current of the heater reaches a level that is approximately ten times as much as the current in a steady state.
The rush current at the heater ON-time naturally flows also to an AC power supply line, wherein an instantaneous voltage drop is caused by the rush current due to impedance of the AC line, with the result that a so-called flicker occurs. The flicker means a flicker of interior lighting equipment due to the instantaneous voltage drop of the AC line. The flicker uncomfortably affects a feeling of a user. Especially, the high-speed laser beam printer and the electrophotographic copying machine requires a high-power heater, and there must be a large influence by this flicker.
For coping with this problem arising from the flicker, as disclosed in, e.g., Japanese Patent Application Laid-Open No. H6-230702, not the low-frequency ON/OFF control by the TRIAC but the high-frequency switching control is adopted. A Field Effect Transistor (FET) is employed as a device for this switching control, and a LC filter circuit is utilized for an output of the switching circuit in order to restrain copy noises.
The switching device such as the FET switches ON/OFF only the current in one direction at a high frequency, and therefore requires a circuit for full-wave-rectifying an AC line voltage. Namely, an AC sine wave pattern is converted into a full-wave-rectified voltage wave pattern, the full-wave-rectified voltage wave pattern is further subjected to switching by the FET, then the wave pattern thereof is corrected by the LC filter, and the wave-pattern-corrected voltage is supplied to the heater. The FET as the switching device, though ON/OFF-controlled at the high frequency, adjusts a peak value or an average value of the voltage wave pattern applied to the heater. Namely, the FET keeps the voltage supplied to the heater to a predetermined value. Then, when the heater is switched ON/OFF, a duty cycle ratio thereof is so controlled as to gradually increase from a low value. The control of the duty cycle at the ON/OFF time is called slow-up control. Under this slow-up control, the peak value or the average value of the full-wave-rectified voltage applied to the heater when switched ON/OFF rises stepwise, and hence there is no excessive flow of the rush current at the ON/OFF time.
Thus, the rush current can be restrained low by performing the ON/OFF control of the switching device operating at the high frequency, thereby obviating the flicker problem.
The laser beam printer and the electrophotographic copying machine are, however, accompanied with a difficulty other than the flicker in order to control the electric power for the heater. This is a restriction of the maximum electric power.
In Japan, the AC line voltage is nominally 100 V (an effective value) for the general interior wiring, and the maximum current per receptacle is determined to be 15 A. Accordingly, in the 100 V wiring, only the electric power of 1,500 W at the maximum can be supplied. Further, in North America, the AC line voltage is nominally 120 V (the effective value), and the maximum current per receptacle is determined to be 13.2 A. Therefore, in the 120 V wiring, only the electric power of 1,584 W at the maximum can be supplied. In EU, the AC line voltage is nominally 230 V, and the maximum current per receptacle is 10 A. Hence, the electric power up to 2,300 W can be supplied.
On the other hand, in the high-speed laser beam printer and electrophotographic copying machine (capable of printing, e.g., 50 sheets per minute), the electric power needed for the heater is as high as 1,000 W. The heater consumes the electric power as much as 1,000 W of the total electric power of 1,500 W. Consequently, all the control of the apparatus must be done by the remaining electric power of 500 W. Moreover, the heater drive circuit has a drive loss, and therefore the electric power utilizable for other than a heater system becomes much less. Still further, the high-speed electrophotographic copying machine involves the use of a glass tube lamp for scanning an image of an original, and a large amount of electric power is consumed for this glass tube lamp. Furthermore, in the high-speed laser beam printer and electrophotographic copying machine, a sheet feeding device and a sheet discharging device (a stacker and a stapler) as options are utilized often together, and hence it is more difficult to restrain the electric power down to totally 1,500 W or under. As a matter of fact, however, power supply lines of approximately 200 V, though existing in Japan and North America, are not widely utilized. Therefore, the apparatuses operable at 100 V and 120 V gain high popularity.
Another problem is that the electric power consumed by the heater has a large dispersion. The electric power consumed by the glass tube heater such as a halogen heater has a large dispersion (which is normally on the order or ±3.5%) depending on lots. Taking this dispersion into account, the electric power must be restrained down to totally 1,500 W or under in Japan. In a case where a resistance value of the heater is low and the electric power consumed rises, if contrived to meet this specified electric power of 1,500 W, it follows that there occurs a 7% decrease at the maximum in the electric power for consumption on such an occasion that the heater resistance value rises and the electric power consumed by the heater is lowered. For example, assuming a fixing unit requiring 1,000 W in a way that takes the heater-related power dispersion into consideration, it follows that the electric power consumed by the heater comes to 1,070 W at the maximum due to a dispersion of the resistance value of the heater. As a result, there occurs a 70 W reduction in the amount of electric power utilizable for other than the heater.
As described above, under circumstances of the power supply voltages in Japan and North America and due to the dispersion in the electric power for the glass tube heater, the high-speed laser beam printer and electrophotographic copying machine have a difficulty to attain the restriction of the maximum electric power. In fact, in Japan and North America, the high-speed machine capable of printing approximately 80 sheets per minute has no alternative but to utilize the 200V power supply.
Accordingly, the high-speed laser beam printer and electrophotographic copying machine is incapable of further improving the printing speed because of the restriction of the total amount of utilizable electric power.